1. Field of the Invention
The present invention relates to a portable telephone set and a portable telecommunication apparatus to be suitably used for a second generation cordless telephone system and, more particularly, it relates to a telephone set and a portable telecommunication apparatus having a function of automatically determining if it is in or out of the service area.
2. Description of the Related Art
With the rapid development of digital technologies in recent years, mobile telecommunication systems using a digital communication scheme are becoming more and more popular instead of those using an analog communication scheme. The mobile telecommunication system necessarily involves exchange of control signals between radio base stations connected to a telecommunication network and portable telecommunication apparatuses belonging to the system. A basic system for realizing high efficient and high quality radio communications can be constructed by using digitized control signals. With the application of digital technologies to the mobile telecommunication system, both the radio base station and the portable telecommunication apparatuses can be downsized so that the portable telecommunication apparatuses may provide an enhanced level of portability.
Thus, the number of subscribers of portable telephone sets which is a kind of portable communication apparatus utilizing digital technologies are remarkably increasing. The digital communication system is free from the problems that conventional analog communication systems are hard to meet the increasing number of subscribers for mobile telecommunication systems because of restricted frequency allocation and have a deficiency of limited confidentiality. Recently, a second generation cordless telephone system utilizing widely such digital technologies has been intensively developed.
In the second generation cordless telephone system, a vast number of channels are created by limiting the effective area of each single portable telephone set to a small zone called micro cell and allocating repeatedly a single frequency band to different subscribers through the time division multiplex so that the limited number of frequency resources available for telecommunications can be effectively exploited. Since each zone (or cell) is made small, only a low output is required for the portable telephone set so that it may consume power at a reduced rate and can be constructed to be compact. Hence the telephone set can operate for a prolonged period and it can use a small size battery.
In the second generation cordless telephone system using portable telephone sets, telecommunication channels are established separately between portable telephone sets and a public radio base station connected to the telecommunication network and between the portable telephone set and a private radio base station set (a base station telephone set) which is the base station of the portable telephone set. A particular service area is assigned to the base station, where it can communicate with its portable telephone sets. The public radio base station can be called by any portable telephone sets whereas the private base station set can be called only by its portable telephone set.
In the second generation cordless telephone system, each portable telephone set can be used as an extension set (child set) of a base station set (parent set) of a conventional telephone system having an automatic answering function. The portable telephone set, on its part, is provided with a remote control function, with which, for example, it can obtain messages stored in the base station set during the subscribers' absence by operating remotely the automatic answering function of the base station set in the home from the portable telephone set.
On the other hand, the base station set can also be accessed from and controlled by a public telephone set. If such is the case, a particular identification number which is used to determine whether the remote operation from the public telephone set is allowed is normally registered in the base station set in advance. Then, the subscriber who owns the base station set and hence knows the identification number and the key number corresponding to a function to be operated by the remote operation can successfully obtain the stored message during the subscriber's absence, by calling the base station set through the public telephone set which can transmit the tone signal and then inputting the identification number to the base station set and operating the key number which corresponds to a function to be operated.
However, in case of a portable telephone set which is used in the second generation cordless telephone system and is also used at home as an extension set of a base station set of a conventional telephone system, when an automatic answering function etc. of the base station set is used by means of a remote control operation, two different modes of operation are required for users to use it, one for remotely controlling the base station set having an automatic answering function outside of home and the other for using it at home. Then, the subscriber has to memorize two different sets of procedures for remotely controlling the base station set in two different modes, demanding troublesome remote control procedures to the subscriber, which are hard for the subscriber to remember.
A portable telephone set is normally provided with a display screen that displays an "OUT-OF THE SERVICE AREA" message when it has moved out of the service area of the public radio base station, telling that no telecommunications channel can be established between them any more. When the portable telephone set returns to the service area, the display is cleared to tell that it can now call the base station or vice versa.
Thus, unless the subscriber carrying the portable telephone set always checks the display screen, the subscriber cannot know if he or she is in the service area of the radio base station whenever the subscriber is moving with the portable telephone set.
As a kind of portable communication apparatus, there is a transceiver which can communicate by directly transmitting and receiving radio signals. However, when the destination transceiver is not in the neighborhood, the transceiver requires the user to call the destination transceiver frequently to determine whether or not the destination transceiver is located within its service area since the service area of the transceiver is normally very small. Thus a serious inconvenience is caused to operate the transceiver.
In the second generation cordless telephone system, each portable telephone set is required to have a transceiver function so that a communication channel may be established between any two portable telephone sets without the radio base station if they are located within their service areas. However, since the antenna power of a portable telephone set of the second generation cordless telephone system is limited as low as less than 10 milliwatts, the service areas of the portable telephone sets would be limited to a narrow range having a radius as small as 50 to 100 meters.
Any known portable telephone set that can operate as a transceiver has to have its transceiver function canceled to return to a normal portable telephone set or move around to enter the service area of the destination telephone set, if it has moved out of the service area of the destination portable telephone set and needs to contact with the destination portable telephone set immediately while it is operating as a transceiver.